Dear Hamid Reza,
Let me try to clarify. The damping of the rigid-body mode does not necessarily indicate that the system is unstable. As described in this forum topic: Learizing Baseline 5MW Wind Turbine with FAST, rigid-body modes show up in MBC3 as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping (i.e., eigenvalues with real values only). That is, each rigid-body mode will introduce an additional mode beyond the number of enabled DOFs and the damping is unphysical.
That said, the rigid-body mode may be physically unstable. As described in my Jan 16, 2013 post in the forum topic you referred to (Linearisation Trim), the rigid-body mode will be unstable (in open loop) if the TSR is below the TSR for maximum Cq at a given pitch angle. In this case, it is the generator (or torque controller) that likely keeps the physical system stable (in closed loop).
Best regards,